Hydraulic tool systems typically employ multiple actuators provided with high-pressure fluid from a common pump. In order to efficiently accommodate the different flow and/or pressure requirements of the individual actuators, these systems generally include a pump having variable displacement. Based on individual and/or combined flow and pressure requirements of the actuators, the pump changes a fluid displacement amount to meet demands.
Typical variable displacement pumps used in hydraulic tool systems are known as swashplate-type pumps. A swashplate-type pump includes a plurality of plungers held against a plunger engagement surface of a tiltable swashplate. A ball-and-socket slipper joint is disposed between each plunger and the engagement surface to allow for relative sliding/pivoting movement between the swashplate and the plungers. Each plunger reciprocates within an associated barrel as the plungers rotate relative to the tilted engagement surface of the swashplate. When a plunger is retracted from an associated barrel, low-pressure fluid is drawn into that barrel. When the plunger is forced back into the barrel by the plunger engagement surface of the swashplate, the plunger pushes fluid from the barrel at an elevated pressure.
The tilt angle of the swashplate is directly related to an amount of fluid pushed from each barrel during a single relative rotation between the plungers and the swashplate. Similarly, based on a restriction of a fluid circuit connected to the pump, the amount of fluid pushed from the barrel during each rotation is directly related to the flow rate and pressure of fluid exiting the pump. Accordingly, a higher swashplate tilt angle of a pump equates to a greater flow rate and/or pressure of the pump, while a lower swashplate tilt angle results in a lower flow rate and/or pressure. Likewise, a higher swashplate tilt angle requires more power from a driving source to produce the higher flow rates and pressures than does a lower swashplate tilt angle. As such, when the demand for fluid is low, the swashplate angle is typically reduced to lower the power consumption of the pump.
Historically, the tilt angle of the swashplate has been controlled by way of one or more actuators located on opposing sides of the swashplate. These actuators are selectively extended against a bottom surface of the swashplate or retracted away from the swashplate to directly tilt the swashplate about a tilt axis toward a desired angle against a spring bias. Although effective, these types of actuators can be expensive, difficult to control, and slow to respond.
A pump having an alternative type of displacement actuator used to vary the tilt angle of a swashplate is disclosed in U.S. Pat. No. 5,564,905 issued to Manring on Oct. 15, 1996 (the '905 patent). In particular, the '905 patent discloses a hydraulic unit including a flat port plate disposed between a stationary head and a rotatable cylinder barrel. An arcuate actuator piston extends from the port plate and is slidably disposed within an arcuate pocket in the head to define an actuator chamber. Similarly, an arcuate biasing piston having a pressure area smaller than the actuator piston extends from the port plate and is slidably disposed within another arcuate pocket in the head to define a biasing chamber. The biasing chamber is continuously communicated with a discharge passage in the head, while the actuator chamber is selectively communicated with a control pressure. When the control pressure exceeds a threshold pressure within the actuator chamber, the port plate is caused to rotate in a counterclockwise direction by the actuator piston. When the control pressure falls below the threshold pressure, the port plate is caused to rotate in a clockwise direction by the biasing piston. By selectively rotating the port plate, an amount of fluid pressure carryover of the pump can be varied, thereby changing a swivel torque acting on a swashplate of the pump. In this manner, the swivel toque can be controlled to vary the tilt angle of the swashplate.
While the pump of the '905 patent may provide for tilt angle control of a pump without the use of conventional swashplate-engaging actuators, it may still be less than optimal. In particular, the arcuate pistons, pockets, and chambers disclosed in the '905 patent may be difficult and expensive to fabricate.
The disclosed pump is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.